The present invention relates to a ballast circuit for gas discharging lamps, such as fluorescent lamps or high intensity discharge (HID) lamps. More particularly, the present invention relates to a ballast circuit having a harmonic compensating circuit.
Fluorescent lamps are becoming increasingly popular for use in homes or offices because of their high operating efficiency as compared to incandescent lamps. Indeed, fluorescent lamps emit light at several times the efficiency of a typical incandescent lamp, and do not generate as much heat as a typical incandescent bulb, thereby conserving radiant energy and eliminating excess heat output.
A typical fluorescent lamp is constructed from a glass tube that contains two electrodes at opposite ends, a coating of powdered phosphor covering the interior of the tube, and small amounts of mercury. The major components of a fluorescent lamp are the bulb, electrodes, fill gas, phosphor coating and a base used to support the external conductors of the electrodes. When energized, the electrodes produce a large potential between which free electrons initiate an arc. The arc generates some visible radiation, but mostly ultraviolet radiation, which in turn excites the phosphor coating causing it to emit light. In this process, the fluorescent effect is caused by the excited mercury vaporized in the arc. A ballast circuit is requited for providing a high and constant voltage to operate the fluorescent lamp. In addition to the high voltage requirement, the ballast circuit is requited to maintain a constant current flowing to the fluorescent lamp. A conventional ballast circuit draws a current from its power supply system, which generates great quantity of harmonic current, pollutes the power supply system, and severely decreases power factor of the ballast. The above-described process of operating fluorescent lamps also applies to gas discharging lamps including HID lamps.
Furthermore, to maintain a constant DC output voltage, the conventional current control circuit typically includes a closed-feedback control loop. However, the closed-feedback design suffers low efficiency problem. For example, when a current control circuit includes a boost converter, the output DC voltage must be higher than the peak value of the input voltage, so that the boost converter can be set in a proper operating condition. Therefore, the constant output DC voltage must be set at a voltage that is higher than the maximum (or peak) value of the input voltage. Consequently, when the input voltage becomes low, the conventional ballast circuit needs to perform a large mount of energy conversion to maintain a constant output DC voltage. The more energy conversion is performed, the more self-loss is from the ballast circuit.
In addition, the output of a conventional design is highly sensitive to the accuracy of electronic components used. In the conventional ballast circuit, the high output voltage is typically controlled by resister sampling and stabilizing power source techniques. These techniques are both highly sensitive to component accuracy. Therefore, higher-grade components are needed for the conventional design and thereby the cost of volume manufacturing of the conventional ballast circuits can be prohibitive.
It is therefore an object of the present invention to provide an electronic ballast circuit that has low cost and high efficiency.
It is another object of the present invention to provide an electronic ballast circuit having an improved line current harmonic compensation and high power factor.
It is still another object of the present invention to provide an electronic ballast circuit providing output voltage that is floating in response to a wide range variations of input voltage.
It is additionally another object of the present invention to provide an electronic ballast circuit capable of self-tuning to the variations in the input voltage.
It is yet another object of the present invention to provide an electronic ballast circuit that may employ lower grade and therefore cheap electronic components.
In order to accomplish the above-mentioned objects, the present invention is directed to a ballast circuit having a harmonic compensating circuit. Specifically, the harmonic compensating circuit performs harmonic compensation in the line current in response to a signal indicative of the voltage difference between the high voltage output and the rectified input voltage (i.e. an error voltage) but without a feedback control loop.
In a preferred embodiment, the harmonic compensating circuit of the present invention employs a novel xe2x80x9cfloatingxe2x80x9d output voltage design such that the voltage difference between the output voltage Vo and a peak or near peak value of the rectified input voltage Vir remains substantially constant. By xe2x80x9cfloatingxe2x80x9d the output voltage Vo with substantially the peak value of the rectified input voltage Vir without using a feedback path from the output voltage Vo, the present invention achieves a high power factor (i.e.  greater than 95%) while being able to reduce the sensitivity of the output to the accuracy of the electronic components used.